1. Field of the Invention
The present invention relates to the field of audio processing.
2. Prior Art
In the prior art there exist circuits for removing excessively strong sibilance from electrical signals representing the human voice, one example of which is the dbx Model 263X, manufactured by AKG Acoustics Inc. This device has two level detectors with logarithmic outputs to receive the input signal to be de-essed. The first level detector receives the signal through a third-order high-pass filter with a cutoff frequency of approximately 5 kHz, while the second level detector receives the signal through a band-pass filter contoured to simulate the equal-loudness curves of the human ear. The difference between the detectors' outputs (representing the log of the ratio of their respective input levels) is summed with a reference DC voltage to preset the threshold of de-essing. This sum is applied to a thresholding circuit that detects an "ess" by providing an output whenever the ratio between the high frequency and loudness-contoured powers in the input signal exceed the preset threshold level. The output of the thresholding circuit is applied to a voltage-controlled amplifier (VCA), which quickly and momentarily reduces the channel gain whenever an "ess" sound is detected.
There also exist circuits for performing program-adaptive equalization, such as that disclosed in my U.S. Pat. No. 5,050,217. In this enhancer circuit, a peak detector receives the input signal through a band-pass filter contoured to simulate the equal-loudness curves of the human ear. The input signal is also passed through a first-order high-pass filter to the input of a VCA. The output of the VCA is added to the original input signal to achieve high-frequency equalization. The output of the VCA is also passed through a second high-pass filter to one terminal of a comparator. The other terminal of the comparator receives the output of the peak detector. When the high-passed output of the VCA exceeds the output level of the peak detector, the comparator fires, charging a leaky integrator whose output is coupled to the gain-determining port of the VCA. The gain of the VCA decreases until the high-passed output of the VCA no longer exceeds the output level of the peak detector, at which point the comparator turns off. Thus feedback ensures that the output level of the VCA (representing high-frequency power to be added to the original signal to create the equalized output signal) is always a fixed ratio of the loudness-weighted peak level of the original signal.